The claim by many authors that Spinitectus inermis (Zeder, 1800), a narrowly specific parasite of European eels Anguilla anguilla (L.), is a rare species is considered at three levels: its geographical range, its frequency of occurrence compared to other eel parasites and its relative abundance in component communities. The parasite is widely distributed in freshwater throughout the European range of the eel but its occurrence is erratic and unpredictable, being known from only 8 countries. Surveys of eel parasites in the United Kingdom and in Continental Europe show that it is present in only 13% of British and 29% of continental localities. This satisfies one of the criteria for rarity. When present, its prevalence ranges from 1.8% to 43.3%, so it can be considered rare in some localities but in a few it may be common and on occasion it may be the dominant species in the gastro-intestinal community. Populations of S. inermis are almost always characterised by high levels of overdispersion, even at low prevalence. The species also displays an ability to colonise a locality following introduction there. Overall it meets many of the criteria of a rare species including a restricted distribution and a low frequency of occurrence and so it can be considered to exhibit diffusive rarity.
This paper challenges two paradigms long held in relation to the ecology of parasites in freshwater systems: (1) autogenic species are poorer colonisers than allogenic ones; and (2) parasites with direct life cycles are more successful colonisers than those with complex life cycles. Using new and existing data for Acanthocephala in freshwater fish from the British Isles, it is suggested that all six species present have been able to colonise and persist successfully, in spite of the supposed limitations of their autogenic life-style. It is proposed that these parasites have overcome these limitations by a variety of means, which apply equally to all species considered. Foremost among these is the utilisation of a migratory fish host as either a preferred or a suitable host in their life cycle, allowing colonisation of new areas and rescue effects in established areas, whilst equally important is the use of a common and widespread crustacean as the intermediate host. In addition, all six species appear to exhibit resource partitioning by host at either or both the larval and adult stages, thus reducing the potential for competition and further facilitating colonisation and survival. This hypothesis is supported by data from previous studies both on acanthocephalans from Europe and North America and on other autogenic parasites. It also provides an explanation for the apparently atypical host utilisation patterns of some acanthocephalan species in areas on the edge of their distributions, notably in Ireland.
The need for more long-term studies on helminth communities was addressed by examining changes in composition and diversity of the intestinal helminth component and infracommunities in eels of a small isolated river over 12 years. Examination of samples over one summer season indicated that single samples were representative of community richness in that year. In 1985 the community was species poor (1 species only) and with zero diversity, but by 1996 it comprised six species and all parameters at both levels indicated that it was the richest community yet reported from eels. This overall increase in diversity was not regular or uniform from year to year: rather, the general trend could be reversed in any year(s) and variation between years was considerable. The occurrence of many species was erratic and the identity of the dominant species varied between years. The helminth species that colonised the river contributed to helminth species richness and had an impact on community diversity. It is believed that the overall increase in diversity reflects the changing conditions in the habitat and catchment, but the erratic fluctuations in parameters may reflect the size of the river and the absence of any rescue effects. The findings are compared with the few other long-term studies and it is suggested that the extent of the variation over long periods makes it very difficult to use helminths as indicators of other than general environmental changes or to detect short-term changes due to such specific sources as pollution.
A series of laboratory experiments was conducted to investigate the possibility of post-cyclic transmission in Pomphorhynchus laevis (Muller, 1776). Rainbow trout Oncorhynchus mykiss (Walbaum) were exposed to P. laevis in naturally infected Coitus gobio Linnaeus, Noemacheiius barhalulus (Linnaeus), Phoxinus phoxinus (Linnaeus) and heuciscus cephalus (Linnaeus) and sacrificed one month alter infection. Post-cyclic transmission was possible from all four species even though they came from three families and differed in respect of their status and suitability as hosts of P. laevis. There was no selection for or against cither sex of P. laevis, parasites grew in the rainbows and they occupied the same, normal site in the intestine of rainbows irrespective of source host. Post-cyclic transmission of gravid parasites could occur from C. gobio but not from L. cephalus. It is believed that this failure to transmit larger parasites of either sex reflects the age and so development of the proboscis bulb of P. laevis and the extent of the host encapsulation response rather than size or stage of maturity per se. Post-cyclic transmission has the potential to be important in nature.